Epoxy resins which have been widely used industrially include the so-called epi-bis type epoxy resins which are produced by reacting bisphenol A with epichlorohydrin.
These resins have advantages, e.g., various products can be obtained, which can vary from liquid to solid state, and they can be cured at room temperatures with polyamines because of the high reactivity of the epoxy resins.
However, cured products thereof are defective in that outdoor durability is poor; electric properties such as anti-tracking property, etc., are poor; and the heat distortion temperature is low, although they do have desirable the characteristics of good water resistance and strength.
Recently, epoxy resins prepared by reacting a phenolic resin or a novolak resin with epichlorohydrin have been used as resins for encapsulating VLSI (very large scale integrated circuit), a light emitting or receiving device, etc.
But, chlorine contained in the resins, typically in an amount of several hundreds ppm, causes the problem of a deterioration of the electric properties of such electronic devices.
Epoxy resins having excellent electric properties and heat resistance, and which do not contain chlorine are known. These include certain alicyclic epoxy resins which are produced by an epoxidation reaction of a compound having a 5-membered or 6-membered cycloalkenyl structure.
The epoxy group in these resins is a so-called inner epoxy group, and curing is usually carried out with acid anhydride by heating.
However, since their reactivity is low, they cannot be cured with polyamines at room temperature, and therefore, the use of the alicyclic epoxy resins has so far been technically restricted.
As alicyclic epoxy resins, those having a structure represented by formula (X-I) or (X-II) are presently used commercially. ##STR1##
(X-I) is used as a heat resistant epoxy diluent, because of its very low viscosity.
However, it has the disadvantage of possessing high toxicity and causes the problem of poisoning upon contact with the skin of the human body. (X-II) contains only a small amount of impurities and has a low color hue, and the cured products produced therewith have a high heat distortion temperature.
However, many epoxy resins suffer from the problem of poor water resistance due to the presence of ester bonds.
In addition, because epoxy resins (X-I) and (X-II) have low viscosity, it is impossible to apply molding systems for solid epoxy resins, such as transfer molding, etc., thereto.
From the above viewpoint, novel alicyclic epoxy resins having oxycyclohexane units and ether bonds were taught in Japanese Patent Examined Publication (Kokoku) Nos. 31493/1989, 10471/1992 (equivalent to U.S. Pat. No. 4,565,859).
However, the above epoxy resins have lower softening temperatures.
In addition, not only the overcoming of the above described problems, but also the number of methods or objects involving the use of epoxy resins have grown, and so have the desired characteristics of such epoxy resins; for example, a demand for epoxy resins having excellent various properties has increased.
From the above viewpoint, and as a result of intensive studies by the present inventors, it was found that softening temperatures of epoxy resins can be readily adjusted in a wide range by incorporating ester units in place of ether units into main chain of the molecule.
Furthermore, unsaturated carboxylic esters having vinyl groups are useful as, for example, curing agents for an acrylic rubber.
Allyl methacrylate is known as a typical unsaturated carboxylic ester having vinyl groups.
However, in the case that allyl methacrylate is used as a curing agent for an acrylic rubber, there is a tendency of easy gelation to because of its high reactivity.
On the other hand, glycidyl (meth)acrylate and 3,4-epoxycyclohexylmethyl (meth)acrylate are known as an unsaturated carboxylic ester having epoxy groups.
Recently in particular, it has been taught that 3,4-epoxycyclohexylmethyl (meth)acrylate can be used as a coating composition to be cured by a cationic curing reaction, for example, in Japanese Patent Unexamined Publication (Kokai) Nos. 108048/1988, 108049/1988 (equivalent to U.S. Pat. No. 4,772,672), (Kokai) Nos. 123814/1989, 123817/1989 (equivalent to U.S. Pat. No. 4,923,945), (Kokai) Nos. 129018/1989, 129019/1989 (equivalent to U.S. Pat. No. 4,927,884), etc.
However, the reaction control is difficult because 3,4-epoxycyclohexylmethyl (meth)acrylate has an epoxy group directly bonded to the cyclohexane ring resulting in high reactivity with the cationic curing agent.
In the above-mentioned prior arts, the problem is solved by using a macromonomer in which 3,4-epoxycyclohexylmethyl (meth)acrylate is radically polymerized with a monomer having silicone atoms, etc.
On the other hand, glycidyl (meth)acrylate often shows a too low reactivity even by combination with various curing agents.
Furthermore, glycidyl (meth)acrylate includes chlorine because of the use of epichlorohydrin as a starting material, whereby cured articles or coating layers therefrom have a tendency of being adversely affected.
From the above viewpoint, and as a result of intensive studies by the present inventors, it was found that the problem can be solved by an unsaturated carboxylic ester or ether composition having epoxy groups.